The golgi-associated PDZ domain protein PIST/GOPC stabilizes the β1-adrenergic receptor in intracellular compartments after internalization

Many G-protein-coupled receptors carry C-terminal ligand motifs for PSD-95/discs large/ZO-1 (PDZ) domains; via interaction with PDZ domain-containing scaffold proteins, this allows for integration of receptors into signaling complexes. However, the presence of PDZ domain proteins attached to intracellular membranes suggests that PDZ-type interactions may also contribute to subcellular sorting of receptors. The protein interacting specifically with Tc10 (PIST; also known as GOPC) is a trans-Golgi-associated protein that interacts through its single PDZ domain with a variety of cell surface receptors. Here we show that PIST controls trafficking of the interacting β1-adrenergic receptor both in the anterograde, biosynthetic pathway and during postendocytic recycling. Overexpression and knockdown experiments show that PIST leads to retention of the receptor in the trans-Golgi network (TGN), to the effect that overexpressed PIST reduces activation of the MAPK pathway by β1-adrenergic receptor (β1AR) agonists. Receptors can be released from retention in the TGN by coexpression of the plasma membrane-associated scaffold PSD-95, which allows for transport of receptors to the plasma membrane. Stimulation of β1 receptors and activation of the cAMP pathway lead to relocation of PIST from the TGN to an endosome-like compartment. Here PIST colocalizes with SNX1 and the internalized β1AR and protects endocytosed receptors from lysosomal degradation. In agreement, β1AR levels are decreased in hippocampi of PIST-deficient mice. Our data suggest that PIST contributes to the fine-tuning of β1AR sorting both during biosynthetic and postendocytic trafficking.

Identification of a novel recycling sequence in the C-tail of FPR2/ALX receptor: association with cell protection from apoptosis

Formyl-peptide receptor type 2 (FPR2; also called ALX because it is the receptor for lipoxin A₄) sustains a variety of biological responses relevant to the development and control of inflammation, yet the cellular regulation of this G-protein-coupled receptor remains unexplored. Here we report that, in response to peptide agonist activation, FPR2/ALX undergoes β-arrestin-mediated endocytosis followed by rapid recycling to the plasma membrane. We identify a transplantable recycling sequence that is both necessary and sufficient for efficient receptor recycling. Furthermore, removal of this C-terminal recycling sequence alters the endocytic fate of FPR2/ALX and evokes pro-apoptotic effects in response to agonist activation. This study demonstrates the importance of endocytic recycling in the anti-apoptotic properties of FPR2/ALX and identifies the molecular determinant required for modulation of this process fundamental for the control of inflammation.

Diacylglycerol kinase α regulates tubular recycling endosome biogenesis and major histocompatibility complex class I recycling

Major histocompatibility complex class I (MHC I) presents intracellular-derived peptides to cytotoxic T lymphocytes and its subcellular itinerary is important in regulating the immune response. While a number of diacylglycerol kinase isoforms have been implicated in clathrin-dependent internalization, MHC I lacks the typical motifs known to mediate clathrin-dependent endocytosis. Here we show that depletion of diacylglycerol kinase α (DGKα), a kinase devoid of a clathrin-dependent adaptor protein complex 2 binding site, caused a delay in MHC I recycling to the plasma membrane without affecting the rate of MHC I internalization. We demonstrate that DGKα knock-down causes accumulation of intracellular and surface MHC I, resulting from decreased degradation. Furthermore, we provide evidence that DGKα is required for the generation of phosphatidic acid required for tubular recycling endosome (TRE) biogenesis. Moreover, we show that DGKα forms a complex with the TRE hub protein, MICAL-L1. Given that MICAL-L1 and the F-BAR-containing membrane-tubulating protein Syndapin2 associate selectively with phosphatidic acid, we propose a positive feedback loop in which DGKα generates phosphatidic acid to drive its own recruitment to TRE via its interaction with MICAL-L1. Our data support a novel role for the involvement of DGKα in TRE biogenesis and MHC I recycling.

A hyaluronan receptor for endocytosis (HARE) link domain N-glycan is required for extracellular signal-regulated kinase (ERK) and nuclear factor-κB (NF-κB) signaling in response to the uptake of hyaluronan but not heparin, dermatan sulfate, or acetylated low density lipoprotein (LDL)

The human hyaluronan (HA) receptor for endocytosis (HARE; the 190-kDa C terminus of Stab2) is a major clearance receptor for multiple circulating ligands including HA, heparin (Hep), acetylated LDL (AcLDL), dermatan sulfate (DS), apoptotic debris, and chondroitin sulfate types A, C, D, and E. We previously found that HARE contains an N-glycan in the HA binding Link domain (at Asn(2280)), and cells expressing membrane-bound HARE(N2280A) bind and endocytose HA normally (Harris, E. N., Parry, S., Sutton-Smith, M., Pandey, M. S., Panico, M., Morris, H. R., Haslam, S. M., Dell, A., and Weigel, P. H. (2010) Glycobiology 20, 991-1001). Also, NF-κB-mediated signaling is activated by HARE-mediated endocytosis of HA, Hep, AcLDL, or DS but not by chondroitin sulfates (Pandey, M. S., and Weigel, P. H. (2014) J. Biol. Chem. 289, 1756-1767). Here we investigated the role of Link N-glycans in ligand uptake and NF-κB and ERK1/2 signaling. HA·HARE-mediated ERK1/2 activation was HA size- dependent, as found for NF-κB activation. HARE(N2280A) cells internalized HA, Hep, AcLDL, and DS normally. No ERK1/2 activation occurred during HA endocytosis by HARE(N2280A) cells, but activation did occur with Hep. Dual-luciferase recorder assays showed that NF-κB-mediated gene expression occurred normally in HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but did not occur with HA. Activation of NF-κB by endogenous degradation of IκB-α was observed for HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but not HA. We conclude that a Link domain complex N-glycan is required specifically for HARE·HA-mediated activation of ERK1/2 and NF-κB-mediated gene expression and that this initial activation mechanism is different from and independent of the initial mechanisms for HARE-mediated signaling in response to Hep, AcLDL, or DS uptake.

Opioid receptor function is regulated by post-endocytic peptide processing

Most neuroendocrine peptides are generated in the secretory compartment by proteolysis of the precursors at classical cleavage sites consisting of basic residues by well studied endopeptidases belonging to the subtilisin superfamily. In contrast, a subset of bioactive peptides is generated by processing at non-classical cleavage sites that do not contain basic residues. Neither the peptidases responsible for non-classical cleavages nor the compartment involved in such processing has been well established. Members of the endothelin-converting enzyme (ECE) family are considered good candidate enzymes because they exhibit functional properties that are consistent with such a role. In this study we have explored a role for ECE2 in endocytic processing of δ opioid peptides and its effect on modulating δ opioid receptor function by using selective inhibitors of ECE2 that we had identified previously by homology modeling and virtual screening of a library of small molecules. We found that agonist treatment led to intracellular co-localization of ECE2 with δ opioid receptors. Furthermore, selective inhibitors of ECE2 and reagents that increase the pH of the acidic compartment impaired receptor recycling by protecting the endocytosed peptide from degradation. This, in turn, led to a substantial decrease in surface receptor signaling. Finally, we showed that treatment of primary neurons with the ECE2 inhibitor during recycling led to increased intracellular co-localization of the receptors and ECE2, which in turn led to decreased receptor recycling and signaling by the surface receptors. Together, these results support a role for differential modulation of opioid receptor signaling by post-endocytic processing of peptide agonists by ECE2.

Differential regulation of GABAB receptor trafficking by different modes of N-methyl-D-aspartate (NMDA) receptor signaling

Inhibitory GABAB receptors (GABABRs) can down-regulate most excitatory synapses in the CNS by reducing postsynaptic excitability. Functional GABABRs are heterodimers of GABAB1 and GABAB2 subunits and here we show that the trafficking and surface expression of GABABRs is differentially regulated by synaptic or pathophysiological activation of NMDA receptors (NMDARs). Activation of synaptic NMDARs using a chemLTP protocol increases GABABR recycling and surface expression. In contrast, excitotoxic global activation of synaptic and extrasynaptic NMDARs by bath application of NMDA causes the loss of surface GABABRs. Intriguingly, exposing neurons to extreme metabolic stress using oxygen/glucose deprivation (OGD) increases GABAB1 but decreases GABAB2 surface expression. The increase in surface GABAB1 involves enhanced recycling and is blocked by the NMDAR antagonist AP5. The decrease in surface GABAB2 is also blocked by AP5 and by inhibiting degradation pathways. These results indicate that NMDAR activity is critical in GABABR trafficking and function and that the individual subunits can be separately controlled to regulate neuronal responsiveness and survival.

Spatially restricted G protein-coupled receptor activity via divergent endocytic compartments

Postendocytic sorting of G protein-coupled receptors (GPCRs) is driven by their interactions between highly diverse receptor sequence motifs with their interacting proteins, such as postsynaptic density protein (PSD95), Drosophila disc large tumor suppressor (Dlg1), zonula occludens-1 protein (zo-1) (PDZ) domain proteins. However, whether these diverse interactions provide an underlying functional specificity, in addition to driving sorting, is unknown. Here we identify GPCRs that recycle via distinct PDZ ligand/PDZ protein pairs that exploit their recycling machinery primarily for targeted endosomal localization and signaling specificity. The luteinizing hormone receptor (LHR) and β2-adrenergic receptor (B2AR), two GPCRs sorted to the regulated recycling pathway, underwent divergent trafficking to distinct endosomal compartments. Unlike B2AR, which traffics to early endosomes (EE), LHR internalizes to distinct pre-early endosomes (pre-EEs) for its recycling. Pre-EE localization required interactions of the LHR C-terminal tail with the PDZ protein GAIP-interacting protein C terminus, inhibiting its traffic to EEs. Rerouting the LHR to EEs, or EE-localized GPCRs to pre-EEs, spatially reprograms MAPK signaling. Furthermore, LHR-mediated activation of MAPK signaling requires internalization and is maintained upon loss of the EE compartment. We propose that combinatorial specificity between GPCR sorting sequences and interacting proteins dictates an unprecedented spatiotemporal control in GPCR signal activity.

Role of AKAP79/150 protein in β1-adrenergic receptor trafficking and signaling in mammalian cells

Protein kinase A-anchoring proteins (AKAPs) participate in the formation of macromolecular signaling complexes that include protein kinases, ion channels, effector enzymes, and G-protein-coupled receptors. We examined the role of AKAP79/150 (AKAP5) in trafficking and signaling of the β1-adrenergic receptor (β1-AR). shRNA-mediated down-regulation of AKAP5 in HEK-293 cells inhibited the recycling of the β1-AR. Recycling of the β1-AR in AKAP5 knockdown cells was rescued by shRNA-resistant AKAP5. However, truncated mutants of AKAP5 with deletions in the domains involved in membrane targeting or in binding to calcineurin or PKA failed to restore the recycling of the β1-AR, indicating that full-length AKAP5 was required. Furthermore, recycling of the β1-AR in rat neonatal cardiac myocytes was dependent on targeting the AKAP5-PKA complex to the C-terminal tail of the β1-AR. To analyze the role of AKAP5 more directly, recycling of the β1-AR was determined in ventricular myocytes from AKAP5(-/-) mice. In AKAP5(-/-) myocytes, the agonist-internalized β1-AR did not recycle, except when full-length AKAP5 was reintroduced. These data indicate that AKAP5 exerted specific and profound effects on β1-AR recycling in mammalian cells. Biochemical or real time FRET-based imaging of cyclic AMP revealed that deletion of AKAP5 sensitized the cardiac β1-AR signaling pathway to isoproterenol. Moreover, isoproterenol-mediated increase in contraction rate, surface area, or expression of β-myosin heavy chains was significantly greater in AKAP5(-/-) myocytes than in AKAP5(+/+) myocytes. These results indicate a significant role for the AKAP5 scaffold in signaling and trafficking of the β1-AR in cardiac myocytes and mammalian cells.

Differential roles of C-terminal Eps15 homology domain proteins as vesiculators and tubulators of recycling endosomes

Endocytic recycling involves the return of membranes and receptors to the plasma membrane following their internalization into the cell. Recycling generally occurs from a series of vesicular and tubular membranes localized to the perinuclear region, collectively known as the endocytic recycling compartment. Within this compartment, receptors are sorted into tubular extensions that later undergo vesiculation, allowing transport vesicles to move along microtubules and return to the cell surface where they ultimately undergo fusion with the plasma membrane. Recent studies have led to the hypothesis that the C-terminal Eps15 homology domain (EHD) ATPase proteins are involved in the vesiculation process. Here, we address the functional roles of the four EHD proteins. We developed a novel semipermeabilized cell system in which addition of purified EHD proteins to reconstitute vesiculation allows us to assess the ability of each protein to vesiculate MICAL-L1-decorated tubular recycling endosomes (TREs). Using this assay, we show that EHD1 vesiculates membranes, consistent with enhanced TRE generation observed upon EHD1 depletion. EHD4 serves a role similar to that of EHD1 in TRE vesiculation, whereas EHD2, despite being capable of vesiculating TREs in the semipermeabilized cells, fails to do so in vivo. Surprisingly, the addition of EHD3 causes tubulation of endocytic membranes in our semipermeabilized cell system, consistent with the lack of tubulation observed upon EHD3 depletion. Our novel vesiculation assay and in vitro electron microscopy analysis, combined with in vivo data, provide evidence that the functions of both EHD1 and EHD4 are primarily in TRE membrane vesiculation, whereas EHD3 is a membrane-tubulating protein.

Functions of class V myosins in neurons

This minireview focuses on recent studies implicating class V myosins in organelle and macromolecule transport within neurons. These studies reveal that class V myosins play important roles in a wide range of fundamental processes occurring within neurons, including the transport into dendritic spines of organelles that support synaptic plasticity, the establishment of neuronal shape, the specification of polarized cargo transport, and the subcellular localization of mRNA.

Agonist-biased trafficking of somatostatin receptor 2A in enteric neurons

Somatostatin (SST) 14 and SST 28 activate somatostatin 2A receptors (SSTR2A) on enteric neurons to control gut functions. SST analogs are treatments of neuroendocrine and bleeding disorders, cancer, and diarrhea, with gastrointestinal side effects of constipation, abdominal pain, and nausea. How endogenous agonists and drugs differentially regulate neuronal SSTR2A is unexplored. We evaluated SSTR2A trafficking in murine myenteric neurons and neuroendocrine AtT-20 cells by microscopy and determined whether agonist degradation by endosomal endothelin-converting enzyme 1 (ECE-1) controls SSTR2A trafficking and association with β-arrestins, key regulators of receptors. SST-14, SST-28, and peptide analogs (octreotide, lanreotide, and vapreotide) stimulated clathrin- and dynamin-mediated internalization of SSTR2A, which colocalized with ECE-1 in endosomes and the Golgi. After incubation with SST-14, SSTR2A recycled to the plasma membrane, which required active ECE-1 and an intact Golgi. SSTR2A activated by SST-28, octreotide, lanreotide, or vapreotide was retained within the Golgi and did not recycle. Although ECE-1 rapidly degraded SST-14, SST-28 was resistant to degradation, and ECE-1 did not degrade SST analogs. SST-14 and SST-28 induced transient interactions between SSTR2A and β-arrestins that were stabilized by an ECE-1 inhibitor. Octreotide induced sustained SSTR2A/β-arrestin interactions that were not regulated by ECE-1. Thus, when activated by SST-14, SSTR2A internalizes and recycles via the Golgi, which requires ECE-1 degradation of SST-14 and receptor dissociation from β-arrestins. After activation by ECE-1-resistant SST-28 and analogs, SSTR2A remains in endosomes because of sustained β-arrestin interactions. Therapeutic SST analogs are ECE-1-resistant and retain SSTR2A in endosomes, which may explain their long-lasting actions.

Redox-regulated cargo binding and release by the peroxisomal targeting signal receptor, Pex5

In its role as a mobile receptor for peroxisomal matrix cargo containing a peroxisomal targeting signal called PTS1, the protein Pex5 shuttles between the cytosol and the peroxisome lumen. Pex5 binds PTS1 proteins in the cytosol via its C-terminal tetratricopeptide domains and delivers them to the peroxisome lumen, where the receptor·cargo complex dissociates. The cargo-free receptor is exported to the cytosol for another round of import. How cargo release and receptor recycling are regulated is poorly understood. We found that Pex5 functions as a dimer/oligomer and that its protein interactions with itself (homo-oligomeric) and with Pex8 (hetero-oligomeric) control the binding and release of cargo proteins. These interactions are controlled by a redox-sensitive amino acid, cysteine 10 of Pex5, which is essential for the formation of disulfide bond-linked Pex5 forms, for high affinity cargo binding, and for receptor recycling. Disulfide bond-linked Pex5 showed the highest affinity for PTS1 cargo. Upon reduction of the disulfide bond by dithiothreitol, Pex5 transitioned to a noncovalent dimer, concomitant with the partial release of PTS1 cargo. Additionally, dissipation of the redox balance between the cytosol and the peroxisome lumen caused an import defect. A hetero-oligomeric interaction between the N-terminal domain (amino acids 1-110) of Pex5 and a conserved motif at the C terminus of Pex8 further facilitates cargo release, but only under reducing conditions. This interaction is also important for the release of PTS1 proteins. We suggest a redox-regulated model for Pex5 function during the peroxisomal matrix protein import cycle.

Single-chain variable fragment albumin fusions bind the neonatal Fc receptor (FcRn) in a species-dependent manner: implications for in vivo half-life evaluation of albumin fusion therapeutics

Albumin has a serum half-life of 3 weeks in humans. This has been utilized to extend the serum persistence of biopharmaceuticals that are fused to albumin. In light of the fact that the neonatal Fc receptor (FcRn) is a key regulator of albumin homeostasis, it is crucial to address how fusion of therapeutics to albumin impacts binding to FcRn. Here, we report on a detailed molecular investigation on how genetic fusion of a short peptide or an single-chain variable fragment (scFv) fragment to human serum albumin (HSA) influences pH-dependent binding to FcRn from mouse, rat, monkey, and human. We have found that fusion to the N- or C-terminal end of HSA only slightly reduces receptor binding, where the most noticeable effect is seen after fusion to the C-terminal end. Furthermore, in contrast to the observed strong binding to human and monkey FcRn, HSA and all HSA fusions bound very poorly to mouse and rat versions of the receptor. Thus, we demonstrate that conventional rodents are limited as preclinical models for analysis of serum half-life of HSA-based biopharmaceuticals. This finding is explained by cross-species differences mainly found within domain III (DIII) of albumin. Our data demonstrate that although fusion, particularly to the C-terminal end, may slightly reduce the affinity for FcRn, HSA is versatile as a carrier of biopharmaceuticals.